A magneto-inductive flow-measuring device includes the following components:                A magnet system, which produces a magnetic field passing through the measuring tube essentially perpendicularly to the measuring tube axis;        at least two measuring electrodes coupled with the medium, the measuring electrodes being of defined rest potential and arranged in a region of the measuring tube lying essentially perpendicularly to the magnetic field; and        a control/evaluation unit, which delivers, on the basis of the measurement voltage induced in the measuring electrodes, information concerning volume- or mass-flow of the medium in the measuring tube.        
Corresponding magneto-inductive flow-measuring devices are available from the company, Endress+Hauser in a plurality of embodiments, such as those offered and distributed under the mark PROMAG.
Described in DE 103 56 007 B3 is a magneto-inductive flow-measuring device providing an added feature. In order to achieve this added feature, an electrical current is applied between a measuring electrode and a reference electrode or a fill-level monitoring electrode. The voltage present in such case between the measuring electrodes is measured and from the ratio of the measured voltage to the applied electrical current, a first resistance value is ascertained. Then, the first measuring electrode is replaced by the second measuring electrode and the above-described method steps are repeated, whereby a second resistance value is available. By forming the difference or quotient of the two ascertained resistance values, a critical coating of one of the two measuring electrodes is detected, when the formed ratio, or difference, lies outside of a predetermined range of values. Disadvantageous in the case of this flow-measuring device is that each of the measuring electrodes is moved out of rest potential by the application of an electrical current, i.e. the rest potential shifts. In order subsequently to be able to perform a correct flow measurement, it is necessary to wait until the equilibrium state has again been reached. Since less measuring time is available, the measuring accuracy of the flow-measuring device is reduced.
EP 0 336 615 B1 discloses an electromagnetic flow-measuring device which permits simultaneous determination of flow of a medium through a measuring tube of the flow-measuring device and the electrical conductivity of the medium. The flow-measuring device includes the components named in the introduction. Beyond the known flow-measuring devices, the solution disclosed in this European publication includes a means for producing from an output signal produced likewise as the flow measurement signal from the two measuring electrodes a conductivity signal representing the conductivity of the medium. For this purpose, in addition to the alternating, rectangularly shaped, exciting signal for the magnet system, a means is provided for producing an electrical pulse which is applied to the magnet system via a corresponding control, in each case, at the beginning of each half cycle of the exciting signal.
U.S. Pat. No. 6,804,613 discloses an electromagnetic flow meter, which can ascertain, besides the information concerning flow, also information for detecting an empty tube, an accretion adhering to the measuring electrode, or the electrical conductivity of the medium flowing through the measuring tube. For this purpose, diagnostic signals are applied, in each case, between one of the two measuring electrodes and a grounding electrode. The corresponding diagnostic signal generators are either constant current supplies or constant voltage supplies, with the diagnostic signal generators using alternating signals, whose frequency is a whole-numbered multiple of the exciting frequency used in the excitation circuit for the magnet system. Additionally, a diagnostic circuit is provided, which synchronizes the excitation frequency of the magnet system with the frequency of the diagnostic signal. Especially, the conductivity of the medium or the formation of an accretion on the measuring electrodes is determined via measurement of the resistance of the measuring electrodes or via measurement of resistance between a measuring electrode and the grounding electrode.
Disadvantageous in the known method is that, here, it is assumed that the measured resistance of the liquid stands in a unique relationship with the conductivity of the medium. In the region of higher conductivities, the measuring range of a conductivity-measuring cell is limited by the phase transition from the measuring electrode to the liquid. The impedance of the phase transition behaves only in the ideal case purely capacitively; in the real case, the impedance of the phase transition has also an ohmic portion. If this portion can no longer be neglected compared to the ohmic resistance of the medium, then the measured conductivity of the medium contains error. As a result, the measuring range of the conductivity-measuring cell has an upper limit.
For the purpose of eliminating this problem, EP 0 990 984 B1 discloses an improved measuring cell for determining electrical conductivity of a liquid medium. Especially, the liquid medium is, in this case, a calibration solution for pH electrodes. Here, the impedance of the measuring cell extending into the liquid is ascertained at least two frequency values of an alternating voltage. From the ascertained impedance values, based on an equivalent circuit, frequency independent parameters and the resistance value, from which the conductivity is ascertained, are determined. The equivalent circuit is composed of a parallel connection of a capacitor representing the capacitance of the measuring cell and an ohmic resistance representing the sought resistance of the liquid within the measuring cell, as well as an electrical component connected in series with the ohmic resistance and having a frequency-independent phase. In the case of each of the at least two frequency values, the real part and the imaginary part of the impedance of the measuring cell are ascertained. From the ascertained values, subsequently, the frequency-independent parameters and the sought resistance are calculated. Then, the calculation of the electrical conductivity is done.